The present invention relates generally to gas turbine engine control, and more particularly to a method and system for model-based compressor control.
Modern Brayton and Ericsson cycle engines, including gas turbine engines for aircraft applications, continue to grow more complex. These engines require sophisticated control systems to handle increasing operational demands at reduced tolerances. Such engine control systems command engine actuators for control parameters such as fuel flow rate and variable engine geometries to achieve desired values of output parameters such as net thrust or engine rotor speed. A variety of control methods are currently used toward this end, including model-based control algorithms using predictive models that relate thermodynamic parameters such as flow rate, pressure, and temperature to input and output variables such as overall thrust, power output, or rotational energy.
Engine control systems are typically provided with a plurality of inputs including both current operating parameters and target parameters. Current operating parameters may include engine parameters such as rotor speeds, engine temperatures, and flow rates, as well as environmental parameters such as altitude and environmental air pressure and flow rate. Some current operating parameters are directly measured, while others may be fixed at manufacture or estimated based on measured parameters. Target parameters may include desired rotor speeds or net thrust values specified according to desired aircraft activities.
In addition to achieving specified target parameters, engine control systems are expected to avoid engine trajectories resulting in engine states that unduly reduce component lifetimes or increase likelihoods of undesired events such as engine surge, compressor stall, or engine blowout. Compressor stability, in particular, is maintained by controlling bleeds and variable stator vane angles to avoid compressor stall or lean blowout conditions.
Engine control systems maintain a stall margin, a minimum distance between a compressor operating point (i.e. compressor pressure ratio and flow) and a predicted stall line corresponding to compressor stall conditions. Conventional systems rely on lookup tables generated offline from steady-state engine models with entries corresponding to expected pressure ratio targets selected to avoid stall conditions by at least a “stall margin,” a tolerance margin chosen to minimize risk of stall. The more accurate and precise the prediction of stall conditions, the narrower the stall margin may be. Improvements in stall margin estimation allow improved engine efficiency by reducing the operating stall margin.